The possible involvement of prolactin on the gastrointestinal handling of calcuium in wistar rats

Abstract

Possible influence of exogenous ovine prolactin (PRL) on the in vivo intestinal calcium (Ca) absorption was investigated in non-pregnant, pregnant, and lactating female Wistar rats. Solution containing 5.0 mM CaCl(,2) and 0.1 uCi (45)Ca was given intragastrically to the control animals and prolactin-treated animals which received intraperitoneal administration of 0.01 or 0.02 mg PRL/100 g BW immediately, or 1 hr before the experiment, and subcutaneous administration of 0.1 or 0.2 mg PRL/100 g BW 24 hr before the experiment. Subsequent plasma (45)Ca contents were used as index of intestinal calcium absorption. In the non-pregnant females, the most appropriate dose of prolactin was an intraperitoneal administration of 0.02 mg PRL/100 g BW 1 hr before the experiment. The plasma (45)Ca contents of control versus PRL-treated rats were 2.09±0.28 VS 4.39±0.61 (p < 0.05), 2.52±0.14 VS 5.62±0.49 (p<0.001), 3.14±0.16 VS 6.54±0.51 (p<0.001), 3.54±0.23 VS 6.22±0.41 (p < 0.001) and 2.98±0.30 VS 5.43±0.33% administered dose (p<0.001) at 5,10,15,30 and 60 min respectively. The intestinal mucosa (45)Ca contents of the PRL-treated rate were significantly decreased in the duodenum and increased in the distal jejunum and ileum. Although female rats showed more responsiveness to prolactin than males, intraperitoneal injection of 0.02 mg PRL/100 g BW 1 hr prior the intragastric administration of test solution also led to a significant increase in calcium absorption in the male rats. The physiological significance of endogenous PRL in the regulation of the intestinal calcium absorption was demonstrated in the pregnant (day 20 of pregnancy) and lactating rats (day 5 of lactation). The experimental groups were given two doses of 3 mg/kg BW of bromocriptine, an inhibitor of prolacting secretion, at 17 hr and 1 hr before the experiment. Bromocriptine treatment significantly decreased the calcium absorption in lactating rats, but not in pregnant rats. The reduced calcium absorption in the former was partially restored by the concurrent administration of exogenous prolactin. The in situ intestinal loop experiments were performed to find out whether the duodenal active transport and the jejunal passive transport of calcium were altered during lactation as compared with nonlactating rats. It was found that the duodenal active transport of calcium was significantly increased while no differences were observed in the jejunal calcium transport of lactating rats. The present investigation, thus, showed that the enhancing effect of PRL on intestinal calcium absorption in rats could be seen immediately after an intraperitoneal injection, suggesting a direct mode of action on the intestine. Endogenous PRL appeared to be important for controlling intestinal calcium absorption during lactation but not during pregnancy. Furthermore, the duodenal active transport of calcium was shown to be significantly increased during lactation.